改动后的封装

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace 简易运算
{
class Pack //封装
{
private int x;
private int y;
private string oper;
private int anster = 0;

public int OP
{
   set
{
    x = value;
  }
    get
  {
    return x;
  }
}
public int Pt
  {
    set
  {
    y = value;
}
get
{
    return y;
  }
}
public string Oper
{
   set
{
   oper = value;
}
get
{
   return oper;
  }
}
    public int Aster
{
    get
  {
    return anster;
  }
}
public int yusuan()
{
    switch (oper)
{
   case "+":
    anster = x + y;
    break;
    case "-":
    if (x > y)
    anster = x - y;
    //else
    // throw new Exception("被减数不能小于减数，既不能出现负数");
     break;
     case "*":
     anster = x * y;
     break;
      case "/":
     //if (y == 0)
  //{
        // throw new Exception("被除数不能等于零！");

   //}
//else
   anster = x / y;
   break;
   }
   return anster;

  }
public int fanhui()
{
    return anster;
}

}
}
<span style="font-size: 18px;">Form1代码:</span>
<br>using System;
using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
using System.IO;

namespace 计算机
{
public partial class Form1 : Form
{
    public Form1()
{
      InitializeComponent();
}
     string path = "F:/";
     public static int count = 0;
     public static int t = 0; 
     public static int right = 0;
     public static int sum;
     int x = 0;
     int y = 0;
private void Form1_Load(object sender, EventArgs e)
{
     Class1 s = new Class1();
       s.text1();
}

private void button1_Click(object sender, EventArgs e)
{
    StreamWriter l1 = File.AppendText("1.txt");
    WriteLine(text1.Text);
    Close();
    StreamWriter l2 = File.AppendText("2.txt");
    WriteLine(comboBox1.SelectedItem.ToString());
    Close();
    StreamWriter l3 = File.AppendText("3.txt");
    WriteLine(text2.Text);
    Close();
    richTextBox1.Text += text1.Text + comboBox1.SelectedItem.ToString() + text2.Text + " ";
x++;
    text1.Text = "";
     text2.Text = "";
      savet.Enabled = true;
     opent.Enabled = true;
      text1.Focus();
}

private void savet_Click(object sender, EventArgs e)
{
      SaveFileDialog TxtSaveDialog = new SaveFileDialog();
      TxtSaveDialog.Filter = "RTF文件(*.RTF)|*.RTF";
        if (File.Exists(path))
{
     this.richTextBox1.SaveFile(path, RichTextBoxStreamType.RichText);
     MessageBox.Show("储存成功", "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
     this.richTextBox1.Clear();
     savet.Enabled = false;
}
else
{
      if (TxtSaveDialog.ShowDialog() == DialogResult.OK)
{

     this.richTextBox1.SaveFile(TxtSaveDialog.FileName, RichTextBoxStreamType.RichText);
     MessageBox.Show("储存成功", "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
     this.richTextBox1.Clear();
      savet.Enabled = false;
      }
   }
}

private void opent_Click(object sender, EventArgs e)
{
     OpenFileDialog TxTOpenDialog = new OpenFileDialog();
     TxTOpenDialog.Filter = "RTF文件(*.RTF)|*.RTF";
     if (TxTOpenDialog.ShowDialog() == DialogResult.OK)
{
    path = TxTOpenDialog.FileName;
    this.richTextBox1.LoadFile(TxTOpenDialog.FileName, RichTextBoxStreamType.RichText);
    savet.Enabled = false;
    opent.Enabled = false;
     MessageBox.Show("打开成功", "信息提示", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
  }
}

private void button2_Click(object sender, EventArgs e)
{
      jishi.Text = t.ToString();
      timer1.Enabled = true;
      timer1.Interval = 1000;
      timer1.Start();
     string[] l1 = new string[100];
    l1 = File.ReadAllLines("1.txt");
        string[] l2 = new string[100];
        l2 = File.ReadAllLines("2.txt");
        string[] l3 = new string[100];
        l3 = File.ReadAllLines("3.txt");
       Box1.Text = l1[0];
       Box4.Text = l2[0];
       Box2.Text = l3[0];
}

private void Box3_KeyDown(object sender, KeyEventArgs e)
{

      //string s = Box4.Text;
        Pack operast = new Pack();
         operast.Pt = int.Pars(Box1.Text);
          operast.Oper = Box4.Text;
        //switch (s)
          //{
          //case "+":
          //sum = int.Parse(Box1.Text) + int.Parse(Box2.Text);

         //break;
         //case "-":
        //sum = int.Parse(Box1.Text) - int.Parse(Box2.Text);

       //break;
        //case "*":
         //sum = int.Parse(Box1.Text) * int.Parse(Box2.Text);

           //break;
          //case "/":
      //sum = int.Parse(Box1.Text) / int.Parse(Box2.Text);

        //break;
         //default:
       //break;
         //}

          operast.yusuan();
      if (e.KeyCode == Keys.Enter)
 {
      if (Box4.Text.ToString() == Box3.Text)
{
     MessageBox.Show("回答正确");
         count++;
          right++;
}
else
{
        MessageBox.Show("回答错误");
          count++;
}
          y++;
        if (y < x)
{
        Box3.Clear();
         string[] ll = new string[100];
           ll = File.ReadAllLines("1.txt");
         Box1.Text = ll[y];
           string[] l2 = new string[100];
         l2 = File.ReadAllLines("2.txt");
          Box4.Text = l2[y];
         string[] l3 = new string[100];
          l3 = File.ReadAllLines("3.txt");
         Box2.Text = l3[y];
}
else
 {
          jishi.Enabled = false;
           Form2 frm = new Form2();
           frm.ShowDialog();
         }
    }
}

private void timer1_Tick(object sender, EventArgs e)
{
    t = t + 1;
        jishi.Text = t.ToString();
}

private void Box1_TextChanged(object sender, EventArgs e)
{

    }
}

<span style="font-size: 18px;">Form2代码：</span><br>using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;
namespace 计算机
{
public partial class Form2 : Form
{
      public Form2()
{
       InitializeComponent();
}

private void Form2_Load(object sender, EventArgs e)
{
      textBox2.Text = Form1.count.ToString();
         textBox1.Text = Form1.right.ToString();
           textBox3.Text = ((Form1.right / (double)(Form1.count)) * 100).ToString() + "%";
}

private void label1_Click(object sender, EventArgs e)
{

       }
}